TANNING is the art of converting into leather the gelatinous parts of the skins of animals by im pregnating them with tannin. The skins chiefly employed are those of bulls, oxen, cows, Stc. which are converted into leather for the soles of shoes, and those of calves, seals, Etc. which are converted into leather for the upper leathers of boots and shoes, saddles and harness. The method of manu facturing these different kinds of leather is as fol lows: I. Method of manufacturing butts or heaviest and stoutest of the bull and ox hides being selected for the purpose, they are allowed to lie in a heap for two or three days, and are then hung up on poles in the smoke house, kept, by means of a mouldering fire, at a temperature somewhat above that of the atmosphere. A slight putrefaction takes place, loosens the epidermis, and renders it easy to separate the hair and other extraneous matter from the true skin, by the fleshing knife, with which it is scraped on a convex board, called a horse. The hides thus cleaned are then raised or steeped in water containing a little sulphuric acid. The effect of the acid is to distend the pores of the skin, and prepare it for the reception of the tanning.
The hides are now removed to a pit, and laid one above another, with a stratum of powdered oak bark below each, and the pit is filled with the tan nin lixivium or ooze prepared from oak bark and water. Here they remain for a month or six weeks, at the end of which they are taken out and replaced with fresh bark and ooze. After remaining three months, the operation is repeated three times or oftener at the same intervals. When they have thus sufficiently imbibed the tannin, they are sus pended in a shed to dry, and when compressed with a steel instrument, and rendered firm and clean by beating, they arc ready for sale, and are used for the thickest sole leather.
2. Method of manufacturing crop hides are immersed for three or four clays in pits containing a mixture of lime and water, being oc casionally stirred up and down. When the extra neous matters have been removed by the flesh knife as above described, they arc worked in water.
They are now immersed into a weak ooze, and gradually removed to other pits with stronger ooze, being moved up and down, or handled, as it is called, once every clay. At the end of a month or six weeks they are put into pits with a stronger ooze and a little powdered bark, and the process is repeated with fresh ooze and bark for two or three months. They are then put into larger vats, called layers, in which they are laid one above another in an ooze of still greater strength, and with a large quantity of good bark interposed. They remain here for six weeks, and they are then taken up and relaid with strong ooze of fresh bark for two months. This process may be repeated once, twice or thrice, according to circumstances, and when it is completed, the skins are taken out, dried, and smoothed as before. The sole leather used in England consists of the crop hides.
3. Method of manufacturing the skins of calves and seals, 4-e.—Hides of this description are kept in the lime pits for ten or fifteen days: the hair, kc. is then removed, and they are steeped in an infusion of pigeon's dung, called a grainer. Here they are frequently tumbled about and taken out and scraped, and at the end of a week or ten days they are re moved into pits containing a weak solution of bark, where they go through the same process of hand ling, Ste. nearly as crop hides, though they are sel dom put down in layers. This operation lasts from two to four months. The skins are then dried, and the currier dresses and bleaches them for the dif ferent purposes to which they are to be applied. For farther information on this subject see " Dr. Macbride's paper, Philosophical Transactions, vol. ix. p. 111," vol. LXVIII, and that for 1803. See also Newton's Journal of the ..1r1s, Jan. 1829, p. 219, for an account of a powerful tanning lixivium used in America, by J. Giles. It is announced in the Journals that M. Rapedius has discovered that 3& lbs. of the bilberry or Nvhortleberry plant, (raccinium myrtillus) will tan a pound of leather which requires 6 lbs. of oak bark. This important experiment has been successfully tried at Treves.